A zoom lens is composed of movable zooming lens groups, movable focusing lens groups, and fixed lens groups. By moving the position of the focusing lens groups, an object can be focused at a certain position, like a film. By moving the position of the zooming lens groups, the size of the image of the object can be adjusted.
In photographic camera systems using charge-coupled devices (CCDs), there are usually two types of zoom lenses in driving the focusing lens groups, i.e., front focus type and inner focus type. In comparison with the front focus type, the inner focus type zoom lenses have higher accuracy and shorter stroke in focusing. Since the focusing lens groups are at rear end of the zoom lens in the inner focus type, the focusing lenses have smaller diameter and lower weight. Therefore, inner focus type is commonly used in camera systems.
Please refer to FIG. 1. A conventional driving mechanism of the zoom lens groups, as disclosed in U.S. Pat. No. 4,920,420, includes a fixed lens group 1, a zoom lens group 6, and a driving mechanism composed of a step motor 3, a reduction gear set 4 and a driving screw 5. The step motor 3 drives the driving screw 5 through the reduction gear set 4, then the zoom lens group 6 is driven by the driving screw 5. In the conventional driving mechanisms, a conversion mechanism, like the reduction gear set 4 and the driving screw 5 in this example, for converting the rotating motion of the motor into linear motion of the zoom lens groups is indispensable. However, the conversion mechanism also brings the problems of power loss and limitation of speed, therefore the focusing speed of the zoom lens is restricted by the driving mechanism. Furthermore, due to the backlash of the conversion mechanism, the accuracy of focusing is reduced, and sharp images cannot be achieved. In addition, the life of the conversion mechanism is reduced due to wear between each of the gears and the screw.